The invention relates to a process for granulating slag, in particular from a blast furnace and/or a smelting reduction plant, in which a granule/water mixture formed during the granulation is fed to a granulation tank and then to a dewatering installation, in which the slag granules are dewatered, the H2S-containing vapors and gases formed during the granulation being at least partially condensed by injection of water in a condensation space which is flow-connected to the granulation tank.
Hot slag coming out of a blast furnace or a smelting reduction plant is converted into granules, for example by rapid cooling and comminution using water. After the granulation, the granule/water mixture flows via a granulation tank or a passage to a dewatering installation, in which the slag sand is dewatered down to approx. 12% and then sold as a finished product.
The steam produced in the course of the granulation process and the sulfur-containing gases, H2S and small quantities of SO2, are generally passed into the atmosphere via a high stack or are precipitated in a condensation tower arranged above the granulation tank.
“Fachbericht Hüttenpraxis Metallweiterverarbeitung” [Specialist Report on Further Processing of Foundry Metals] (Vol. 20, No. 10, 1982, pp. 744-746) describes a process for producing slag granules, in which a vapor condenser can be installed in the stack and allows condensation of the vapors including a large proportion of condensable pollutants.
A process of the type described in the introduction is known from DE 35 11 958 C. In this case, the gas streams, comprising a steam/flue vapor mixture, the term flue vapor being understood to mean both air and pollutants, such as H2S and SO2, are passed in a closed circuit and precipitated in a condensation tower using water containing calcium oxide.
However, one drawback of this process is that H2S and SO2 are only precipitated using water down to defined residual concentrations.
The quantity of air which is sucked in or introduced into the system in some other way and the quantities of H2S produced fluctuate very considerably over the course of a tapping and from tapping to tapping as a function of slag rate, slag analysis, water circulation quantity, water temperature, wind speed, wind direction, shape and design of the granulation tube and other factors. The air introduced into the system leads to a slight superatmospheric pressure in other regions of the plant, in accordance with DE 35 11 958 C, and passes to atmosphere via granule ejector openings and other openings and via extractor hoods. However, the harmful gases also escape with the air into the atmosphere in an uncontrolled way in concentrations which are above the permitted limits.
According to another process, described in U.S. Pat. No. 5,540,895 A, the sulfur-containing flue gases are subjected, in a dedicated device in the condensation tower, to a chemical gas scrub by means of injection of an alkaline aqueous solution before they are discharged to atmosphere. However, this requires an additional chemical installation and concomitant consumption of chemicals.